The present invention relates to small clearance retention apparatus, and more particularly to a small clearance retention apparatus capable of retaining uniformly a small clearance between cylindrical rotating member and a specific or cooperating member, such as the clearance between a donor drum and a recording electrode, extending over the required full length of such members.
As an example of an apparatus which requires that the small clearance between a cylindrical rotating member and a specific member be maintained constant, there is an electrostatic recording apparatus employing a drum and a recording electrode facing the drum. In the apparatus, the small clearance between the drum and the recording electrode has to remain constant.
In the electrostatic recording apparatus, there are two types, namely a direct type and a transfer type.
In the direct type electrostatic recording apparatus, there are disposed a donor drum whose surface is uniformly covered with conductive magnetic toner and which is rotated at a predetermined constant speed, and a recording electrode in which numerous recording styluses are aligned in the axial direction of the donor drum, with a predetermined clearance between the donor drum and the recording electrode, and a recording paper is caused to pass between the recording electrode and the donor drum while applying image information signals from a signal generating apparatus to the recording electrode. In accordance with the magnitude of the applied voltage, toner on the drum is attracted to the recording sheet, so that an image is reproduced. In such an apparatus, when the above-mentioned clearance is too small, the recording sheet is brought into contact with the toner on the drum, causing toner deposition on the background of the reproduced image. On the other hand, when the clearance is too great, the electric field in the clearance becomes so weak that the image density of the reproduced image becomes low. Accordingly, when the clearance is changed during operation, the image density of the reproduced image becomes uneven. Therefore, the clearance has to be always maintained constant.
In the case of a transfer type electrostatic recording apparatus, a dielectric drum is used as the drum, and in operation, the surface of the dielectric drum is charged uniformly in advance and the charge is then neutralized selectively by applying charges thereto in accordance with the image signals from the recording electrode or a latent electrostatic image is formed by applying charges to the surface of the dielectric drum from the recording electrode in accordance with the image signals and the thus formed latent electrostatic image is developed with toner and the toner image is then transferred to the recording sheet.
In the conventional clearance retention apparatus for use in such recording apparatus, spacers, such as rollers, are disposed on the opposite sides of the recording electrode in the axial direction of the drum, and the spacers are brought into contact with the outer peripheral surface of the drum, with the outer peripheral surface of the drum set as a standard surface, whereby the clearance is maintained. In the case where the conventional spacers are fixed to the opposite ends of the recording electrode or disposed rotatable, there is a danger that a perdetermined clearance cannot be obtained uniformly over the necessary longitudinal length thereof due to the eccentricity of the drum or some other errors which may occur during the production thereof. Furthermore, it is extremely difficult to adjust the clearance.
In case no foreign material, such as powder toner, exists in the contact area between the spacers and the drum, the clearance may be maintained constant. However, in the case of the above-mentioned direct type recording apparatus, since the donor drum bears the powder toner on the whole peripheral surface thereof, the powder toner may be scattered and the scattered toner is apt to be deposited on the contact area between the spacer members and the donor drum. When the toner is deposited on the contact area between the spacers and the donor drum, toner is caused to adhere to the spacers or to the outer surface of the donor drum by the pressing force between the spacers and the donor drum. In this condition, the surface of the spacers or that of the donor drum becomes uneven, and in particular, a complete cylindrical surface cannot be obtained on the outer peripheral surface of the donor drum, resulting in that the clearance cannot be maintained constant or the outer peripheral surface of the donor drum is scratched.
In the case where the spacers are rollers which rotate along the outer peripheral surface of the donor drum, the powder toner is scattered, entering the bearing portions of the rollers. This hinders the correct rotation of the rollers and causes abrasion of the bearing portions and abnormal noises during the rotation of the rollers.